In precision systems that perform operations on objects such as workpieces and the like, the object is placed on, held by, and moved as required by a stage assembly that produces controlled motion of the object relative to a tool, optical system, energy source, or other implement that performs the operation(s) on or relative to the object. Such motion can be achieved by any of various actuators. Linear motors have become favored for this purpose due to their wide range of motion, accuracy, precision, reliability, and simplicity, but other types of motors may also or alternatively be used, depending upon the particular stage assembly and the particular type of precision system. For example, planar motors are now being considered for use in stage assemblies used for holding large microlithographic substrates.
A well-known example of a precision system is a microlithography system used for fabricating microelectronic components, displays, microprocessors, RAM memories, and other devices. Some of the stage assemblies used in microlithography systems have very large movable masses. By Newton's Third Law of motion, motor force to produce acceleration or deceleration of the movable mass (principal mass) of the stage in a particular direction produces an equal-magnitude but opposite-direction reactionary force. To absorb these reactionary forces, conventional stage assemblies include counter-masses that move synchronously with (but in the opposite direction to) corresponding motion of the movable mass of the stage itself. I.e., a counter-mass (CM) in a stage assembly is used to absorb at least most of the reaction force produced by corresponding motion of the movable stage mass, thereby reducing transmission of components of the reaction forces to the stage assembly, to structure supporting the stage assembly, or to the floor supporting a precision system including the stage assembly.
A schematic diagram of a conventional stage assembly 100 is shown in FIG. 1. The stage assembly 100 includes a base frame 102, a counter-mass 104, and a movable stage mass 106. The counter-mass 104 is supported by air bearings 108 relative to the base frame 102 so as to allow the counter-mass to move in the x-y plane relative to the base frame 102 substantially without friction. Mounted to the surface of the counter-mass 104 is a stage motor 110 to which the movable stage mass 106 is mounted. Actuation of the stage motor 110 causes corresponding motion of the movable stage mass 106 relative to the counter-mass 104, accompanied by corresponding reaction motion of the counter-mass relative to the base frame 102. Coupled between the counter-mass 104 and the base frame 102 are counter-mass trim-motors 112a, 112b, 114. In the configurations shown, there are two x-direction trim-motors 112a, 112b and one y-direction trim motor 114. The y-direction trim-motor 114 controls the y-position of the counter-mass 104. The two x-direction trim motors 112a, 112b not only control the x-position and x-direction movement of the counter-mass 104, but also control yaw (denoted θz or Tz) of the counter-mass 104 relative to the base frame 102.
As suggested by FIG. 1, the counter-mass 104 in a stage assembly 100 can be massive, depending upon the corresponding movable stage mass 106 and depending upon the displacement of the counter-mass that can be accommodated relative to the corresponding displacement of the movable stage mass. For example, in some stage assemblies the counter-mass 104 is approximately 10× more massive than the movable stage mass 106. To ensure that movement of the counter-mass 104 is truly reactive to corresponding motion of the movable stage mass 106, the stage motor(s) 110 is mounted to the counter-mass. Thus, motion of the movable stage mass 106 in a particular direction causes corresponding motion of the counter-mass 104 in the opposite direction.
Since the stage motor(s) 110 is mounted to the counter-mass 104, the counter-mass usually has electrical wires, cables, and coolant tubes 116 connecting the counter-mass to components and assemblies located elsewhere in the precision system. As the movable stage mass 106 accelerates and decelerates during normal motion, the corresponding motion of the counter-mass 104 relative to the base frame 102 can cause the pendant wires, cables, and tubes 116 to move relative to the counter-mass, which disturbs the counter-mass and needs to be compensated by the trim-motors.
The trim-motors 112a, 112b, 114 control motion of the counter-mass 104. Since the trim-motors 112a, 112b, 114 are not intended to supply all the energy consumed in moving the counter-mass 104, they typically consume low power compared to the stage motor(s) (hence the name “trim” motor). Also, due to space and other limitations, trim-motors 112a, 112b, 114 are normally not located on the center of gravity (CG) of the stage assembly 100. For this reason, the forces exerted by the trim-motors 112a, 112b, 114 should be as low as possible for accomplishing their tasks so as to reduce transmission of vibrations and other forces elsewhere in the system and beyond.
Nevertheless, in certain situations a trim-motor 112a, 112b, 114 receives power in excess of its power rating, resulting in a “saturation” condition. A counter-mass 104 to which a saturated trim-motor is coupled tends to be unstable, which can be a source of instability elsewhere in the system.